This disclosure concerns specialized methods for testing multi-chip packages in order to detect short-circuits on internal components without the need to open the part involved. This method can also be used to test parts on circuit boards.
With the continued expansion of digital technology, more and more attention is being placed and developed on the digital circuitry components which are often found in multi-chip packages. Because of the miniature nature of the components within the multi-chip packages, it is essential that operational testing be effectuated in order to ensure the workability and reliability of a given package. These packages are actually manufactured using multi-layer printed circuit board elements that have materials which could be combustible.
Even after the normal testing methods have been used on the multi-chip packages, it was often found that some of the package parts would actually burst into flames right on the boards of the customers"" product. The fact that there were certain bad parts in the packages which could fail with later use was not easy to detect because, for example, in order to detect a possible short-circuit within the package it was normally necessary to power-up the package and run the risk of burning the entire part.
The short circuits that cause parts to burst into flames were often found on the power bus and because of the already-low resistance of the power bus, it was very difficult, if not impractical, to detect any potential shorts.
It was often more difficult to detect or test the parts which were already placed on the boards. For example, there may be four packages on each board (multi-chip packages), plus also the extra components already on the printed circuit board. It was most desirable to conceive and find some method or system for testing these multi-chip packages before they were delivered for us to a customer in order to ensure reliability and safety factors which might be involved.
As a result, it was conceived that there was the possibility of monitoring the resistance (across the power bus) versus the temperature on a multi-chip package, which could possibly provide a way to detect potential internal shorts even while the packages were mounted on the printed circuit boards. It had been found that the short-circuitry on the internal parts were a result of poor contacts caused by excessive conductive epoxy which would later become short circuits when exposed to heat. It was then seen that monitoring the power bus resistance versus the temperature of a multi-chip package, here could be a way to detect potential short-circuits even when the packages were mounted on printed circuit boards.
It was understood that the normal bus resistance offered a rather linear change against temperature. It was expected that poor or shorted contacts would have an erratic behavior when exposed to heat and therefore, it was possible that by monitoring the total resistance of the power bus that any non-linear component should then be detectable.
As a result of a devised method, it was then found possible to detect short-circuits on components that were normally conceived to have an undetectable effect on the power bus. Thus, just about any component connected directly or indirectly between the internal power bus and ground would have an influence on the power bus resistance to ground and especially so if the component was shorted directly or intermittently by the epoxy used to mount the component. Thus the power bus to ground resistance could indicate a problem at any given temperature of the package.
As a result of the above concepts, the multi-chip circuit package components on the printed circuit boards could be tested without the need to remove them from the printed circuit board, thus saving much time and expense.
A Device Under Test (DUT) such as a multi-chip module is placed adjunct to an aluminum temperature Transfer Block. The aluminum Transfer Block is placed against a Peltier Thermal Electric Module, which is used as a heat pump and utilizes what is called the xe2x80x9cPeltierxe2x80x9d effect to move heat, as if it were a thermal electric cooler. The Peltier Thermal Electric Module is attached to a heat sink and fan.
A digital multi-meter is connected to a test socket for sensing the resistance occurring between a power bus (VCC) and ground, while a temperature meter is connected to the aluminum temperature Transfer Block. A programmable power supply controlled by a computer program is connected to regulate the Peltier Thermal Electric Module and a computer-controlled fan power supply regulates the fan above the heat sink above the Thermal Electric Module.
A personal computer is used to control the above modules in order to cycle an increasing temperature ramp followed by a decreasing ramp. A meter is used to read the Vcc to ground resistance as a function of the temperature change that occurs.
As a result of cycling the voltage-to-ground resistance and reading-out the relative temperature ramp changes, there is found to be a distinctive graph pattern between a module which is normal in operation and a module which has internal intermittent or complete electrical shorts or has a high potential of becoming inoperative.